This invention relates to a foil pressure sensor of the type which can be tested, and has two laminated foil carrier layers, one of which is coated with a semiconductor polymer and the other has strip conductors arranged in a comb pattern.
Such a foil pressure sensor is disclosed in German patent document DE 42 37 072 C1, and is illustrated in FIG. 3a, as a strip conductor diagram. The foil pressure sensor is constructed of two carrier foils that are made of a thermoplastic material and laminated together. The first is coated with a semiconductor polymer, and the second is coated with two adjacent strip conductors which can each be bonded on both ends. Within spatially bounded areas which form sensor elements, the strip conductors have comb structures which mesh with each other, but do not contact each other. When the sensor element is loaded by a normal force, the semiconductor material switches adjacently arranged, more or less parallel, contact fingers of the comb structures, as a function of the force. Electric resistance between the two strip conductors therefore decreases as the pressure force increases.
This foil pressure sensor is conceived for safety-relevant applications in a motor vehicle, particularly for detecting seat occupancy. Each interruption of a strip conductor as the result of mechanical damage or a short circuit between the strip conductors can be detected by means of a simple continuity check, which measures the electric resistance of the strip conductors. The continuity check can, for example, take place in the course of an automatic testing sequence at every start of the vehicle or at predetermined time intervals. By means of the special construction of the contact fingers, which form the comb structure, as strip conductor loops, the contact fingers are fully included in the testing.
The foil pressure sensor of the above-mentioned type, illustrated in FIG. 3a herein, represents a further refinement of the simple foil pressure sensor illustrated in FIG. 3b, which may be derived, for example, from German patent document DE 30 44 384 A1. In this case, the comb structure is implemented by stub lines branching vertically off a main branch.
If a large number of sensor elements are connected in series, a large overall length, and therefore large strip conductor resistances, results in the case of the foil pressure sensor known from the above German patent documents, due to the meandering branch-off-free construction of the strip conductors. From the standpoint of resistance to interference and electromagnetic compatibility (EMV), however, it is desirable to construct the overall system of all pressure-sensitive elements to have a low impedance. For this reason, the semiconductor polymer layer is dimensioned to be of low impedance. However, in this case, because of overlapping resistance ranges, the result of a line continuity check in which the sum of the two specific resistances is measured, cannot be distinguished from the case in which several pressure sensors are acted upon by a maximal pressure. Operational testing in the loaded condition is therefore not possible.
This can be remedied by wider construction of the strip conductors and by reducing the number of contact fingers per sensor element. This would have the result, however, that, on the one hand, the sensor elements become larger and, on the other hand, they become less sensitive. In diverse applications in which, for example, mechanical prestresses and torsional influences could result in incorrect measurements, an enlargement of the sensor surfaces is not desirable. Furthermore, when the foil pressure sensor is used for seat occupancy recognition, a plurality of large-surface sensor elements would impair the air permeability of the seat.
It is an object of the invention to provide a foil pressure sensor which has a low impedance, has a sensitivity which is as high as possible for a particular surface, and that the same time is easily checked.
This object is achieved by the foil pressure sensor according to the invention which differs from the foil pressure sensor of the type described above in that, in addition to the contact fingers constructed as strip conductor loops, the sensor element also has contact fingers which are constructed as stub linesxe2x80x94that is, individual conductors which have a narrower width than the main strip conductors, branch separately therefrom, and are not part of the conductive path formed by the conductor loops. The stub lines require less space than the strip conductor loops so that, while the surface is the same, the sensitivity is increased if one or several strip conductor loops are replaced by stub lines. At the same time, by leaving out strip conductor loops, the strip conductor resistance is reduced so that the desired low-impedance design of the strip conductor structure is achieved.
The sensor element according to the invention also has a facility to be tested which is sufficient in practice. That is, an interruption of an individual contact finger caused by a thin crack, as a rule, does not significantly impair the sensor element because, in the case of a pressure load, a thin crack is in most cases electrically bridged by the semiconductor material. Thus, thin, individually occurring cracks can be tolerated. Therefore, contrary to the prior art described above, it is no longer necessary that each individual contact finger be testable.
A rough ability of the sensor element to be tested is still required and, according to the invention, is ensured by a few contact fingers constructed as strip conductor loops In a continuity check, such an arrangement will recognize any larger crack of the dimension of the sensor element and of a thickness which is not bridged in the case of a pressure load.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.